1. Field of the Invention
The present invention relates to a method for data communication between computational equipment sharing a backplane bus; and, in particular, relates to computational equipment which are part of network under the SONET (Synchronous Optical Network) communication standard, using mainly optical fibers as data transmission media.
2. Discussion of Related Art
Optical fibers provide a high bandwidth medium for data transmission. Consequently, optical fibers have found applications in many computer networks, including those used in digital telephone systems. To allow a uniform interface for voice and computer equipment on an integrated voice and computer network using optical fibers, American National Standards, Inc. adopted a standard, known as SONET (Synchronous Optical Network). The SONET standard is described in "American National Standard for Telecommunications--Digital Hierarchy--Optical Interface Rates and Formats Specification (SONET)" ("SONET document"), which is hereby incorporated by reference in its entirety. The SONET document defines a hierarchy of data formats to support a layered communication architecture, which comprises the photonic, section, line and path layers. A schematic model of the layered architecture is provided in FIG. 1. Each of these layers, except the photonic layer, builds on services provided by the next lower layer.
The basic data unit of the SONET standard is represented by a frame, called the STS-1 frame, consisting of 90 "columns" and 9 "rows" of 8-bit bytes. The STS-1 frame is shown graphically in FIG. 2. Under the fixed transmission rate, the STS-1 frame is transmitted in 125 microseconds. Under the SONET standard, as shown in FIG. 2, data of an STS-1 frame are transmitted row by row, and from left to right. In each byte, the most significant bit is transmitted first.
To support the layered architecture, the first three columns of the STS-1 frame are used for carrying transport overhead information, and the remaining 87 columns of the frame, known as the STS-1 Synchronous Payload Envelope (SPE), carry the data to be transported. Path layer overhead are also carried in the STS-1 SPE. FIG. 3 shows the allocation of the transport and path overheads in the STS-1 frame. A description of each of the overhead bytes is provided in the SONET document and is therefore omitted from this discussion.
The SONET standard also defines (i) data formats which are each smaller than an STS-1 frame and transported within the STS-1 SPE, called virtual tributaries (VT); and (ii) data formats, designated as STS-N frames (where N is an integer), which are each larger than a STS-1 frame. An STS-N frame is formed by byte interleaving N STS-1 frames. The counterparts of the STS-1 and STS-N data formats in the optical fibers are called OC-1 and OC-N (optical carrier level 1 and optical carrier level N) respectively. OC-1 and OC-N are obtained by optical conversions of the respective STS signals after scrambling.
A rough description for each of the layers in the SONET architecture is provided here to facilitate understanding of the present invention. The photonic layer provides transport of bits at a fixed bit rate (51.84 megabits/second) across the physical medium, i.e. the optical fibers. The main function of the photonic layer is the conversion between the STS signals and the OC signals.
The section layer deals with the transport of an STS-N frame across the physical medium. In this layer, framing, scrambling, section error monitoring are provided. An equipment which terminates in the section layer reads, interprets and modifies the section overhead bytes of the STS-1 frame.
The line layer deals with the reliable transport of the path layer payload. A path is a basic unit of a logical point-to-point connection between equipment providing a service on the network. More than one path layer payload, each typically having a data rate less than the STS-1 basic data rate, can share an STS SPE. The line layer synchronizes and multiplexes for the path layer. The overhead bytes for the line layer include overhead involved in maintenance and protection (i.e. error recovery and redundancy) purposes. An equipment which terminates in the line layer reads, interprets and modifies the line layer overhead bytes of the STS-1 frame.
The path layer deals with the transport of services between path terminating equipment. Examples of such services include synchronous and asynchronous DS-1 services and video signals. The main function of the path layer is to map the services into the format required by the line layer.
An architecture, which is found in many computer systems, consists of a backplane bus shared by the computational units of the computer. This backplane bus is the means by which computational units on the backplane communicate local system functions, including such functions as diagnostics, or integration of a back-up unit in case of a failure. In many systems, e.g. a telephone switch or multiplex equipment which transmits and receives pulse code modulation (PCM) data, data communication with computers outside of the backplane are performed on a separate medium from the local data traffic. In such a system, especially one required to service data traffic at the bandwidth of the SONET standard, the communication protocol used on the backplane bus can be an important factor in achieving the fast response time necessary to support data transactions such as protection switch requests, or dynamic bandwidth assignments.
For telephone switches, a protocol on the backplane bus based on a polling algorithm is inherently slow. While a protocol based on an interrupt protocol may satisfy the bandwidth requirement, such protocol requires a large number of interrupt and arbitration lines. Other protocols, such as the distributed arbitration techniques used in MultiBus and NuBus systems, are sufficiently "fair" in terms of providing equal access to units requesting the use of the bus. However, these other protocols do not provide the first-in-first-out behavior when bus requests from the various computational units on the backplane arrive in a non-predictable manner, and require too many additional signals to the backplane.
Other desirable qualities lacking in the prior art protocols, include ease of implementation, a unified bus for both data and local control traffic, and flexibility in message format and message lengths.